OFDM is a data transmission scheme where data is split into a plurality of smaller streams and each stream is transmitted using a sub-carrier with a smaller bandwidth than the total available transmission bandwidth. FIG. 1 shows a graphical representation of orthogonal sub-carriers in OFDM. The efficiency of OFDM depends on choosing these sub-carriers orthogonal to each other. In other words, the sub-carriers do not interfere with each other while each carrying a portion of the total user data.
OFDM system has advantages over other wireless communication systems. When the user data is split into streams carried by different sub-carriers, the effective data rate on each subcarrier is much smaller. Therefore, the symbol duration is much larger. A large symbol duration can tolerate larger delay spreads. In other words, it is not affected by multipath as severely. Therefore, OFDM symbols can tolerate delay spreads that are typical in other wireless communication systems, and do not require complicated receiver designs to recover from multipath delay.
As shown in FIG. 2, splitting the data stream into multiple parallel transmission streams still keeps the basic user data rate the same. Since each symbol duration increases proportionally, any delay spread is proportionally smaller. In practical implementations, the number of subcarriers is from 16 to 2,048.
Another advantage of OFDM is that the generation of orthogonal sub-carriers at the transmitter and receiver can be done by using inverse fast Fourier transform (IFFT) and fast Fourier transform (FFT) engines. Since the IFFT and FFT implementations are well known, OFDM can be implemented easily and does not require complicated receivers.
FIG. 3 is a block diagram of an exemplary OFDM transmitter and receiver. The heart of the transmitter and receiver are IFFT and FFT blocks. IFFT and FFT operations are mathematically almost the same. Therefore, a single computation engine is typically used for both IFFT and FFT operations.
For the benefits that OFDM provides, (i.e., simpler implementation, resistance to larger delay spreads, and efficient use of the spectrum), OFDM is one of the preferred wireless transmission schemes today. It is used in WLAN air interface such as 802.11a, WMAN such as 802.16, and it is part of many wireless communication standards.
Multiple-input multiple-output (MIMO) refers to the type of wireless transmission and reception scheme where both a transmitter and a receiver employ more than one antenna. FIG. 4 shows such a MIMO transmitter and receiver. A MIMO system takes advantage of the spatial diversity or spatial multiplexing and improves signal-to-noise ratio (SNR) and increases throughput.
There are primarily two types of MIMO systems. One type of MIMO system maximizes transmission data rate by taking advantage of the parallel transmissions with MIMO. An example of this type of MIMO scheme is the BLAST system. In this type of a system, the data stream is split into multiple parallel streams and sent across the air interface in parallel. Using a successive interference canceller (SIC) type detector, the receiver separates and collects all parallel streams. Therefore, the effective data rate over the air is increased.
Another type of MIMO system is Space-Time Coding (STC). An STC system provides a much more robust link and therefore can support higher signal constellations. In other words, STC increases the data rate over the air interface by increasing the signaling order, and therefore increasing the effective data rate over the air. An example of STC for a 2×2 MIMO is the so called Alamouti codes.
One of the techniques for increasing the efficiency of OFDM is “waterpouring” and refers to the way that the transmit power of each sub-carrier in OFDM is selected. FIG. 5 shows a typical waterpouring process. The transmitter obtains the channel estimation (step 1), inverts it (step 2), and allocates power to the corresponding sub-carriers starting from the lowest point until the total transmit power is reached (step 3). In order to implement waterpouring, the channel gain information across the transmission band should be known at the transmitter. The receiver may send channel estimation information back to the transmitter in a closed loop manner, or the transmitter can infer the channel from the signals received from the other side.
FIG. 6 shows a block diagram of a prior art MIMO system, such as VBLAST, where data is converted to parallel and transmitted over multiple antennas.
FIGS. 7 and 8 show a prior art scheme for controlling transmit power or modulation and coding scheme per antenna basis. In prior art, transmit power or modulation and coding scheme are determined in accordance with average channel gain or other metric. This scheme introduces flexibility by allowing the transmitter to allocate transmit power or modulation and coding scheme differently to different antennas based on channel response seen at the receiver for each transmit antenna.
FIG. 9 is a block diagram of a prior art system for operation in CDMA based systems known as PARC (per antenna rate control). This scheme transmits using the full transmit bandwidth from each antenna, as typical of any CDMA system. The prior art systems only address per antenna rate control, and not well suited for OFDM application since they are not making use of the sub-carrier level resource allocation available in OFDM.
FIG. 10 is a block diagram of another prior art system, called S-PARC (selective per antenna rate control). This scheme transmits using the full transmit bandwidth from each antenna, as typical of any CDMA system.
Prior art systems are not capable of taking advantage of the subcarrier level resource allocation that OFDM enables. The prior art system adjusts transmit power for each antenna transmission according to average gain across the band that the receiver sees from each transmit antenna. Therefore, the prior art systems are not suitable for OFDM where sub-carrier level resource control is available.